The invention relates to a short circuit and overload protection circuit for output stage transistors.
In order to prevent damage to, output stage transistors, they must not exceed a certain predetermined power dissipation in their operating range. When the load to be driven is short-circuited or when for some other reason (viewed as an equivalent circuit) the load assumes a low resistance, for example, through leakage currents, etc., the collector current of the output stage transistor increases in an inadmissible manner. This also involves an increase in its collector emitter voltage, which results in an inadmissible rise in its power dissipation and, consequently, damage to or destruction of the output stage transistor.
In order to limit the collector current of the output stage transistor to an admissible value, a resistor is normally connected in series with the switching path of the transistor and the consumer. The disadvantage of this arrangement is that in the event of a short circuit, the output stage transistor conducts current. Also, in many applications, the current limiting resistor causes an inadmissibly high voltage drop, with the result that electrical consumers no longer obtain the full operating voltage.
A protective circuit for the output stage transistor wherein the latter is switched off in the event of a short circuit and, therefore, does not heat up, is known from German Auslegeschrift No. 2 310 448. This protective circuitry does, however, require a relatively expensive Zener diode and a measuring resistor which is connected in series with the switching path and reduces the voltage for the consumer.